Ion mobility spectrometry (IMS) is an important technique for the detection of narcotics, explosives, and chemical warfare agents because of its high sensitivity and amenability to miniaturization for field-portable applications. In IMS, gas-phase ions migrate in a drift tube in the presence of a constant electric field. Ions are separated by differences in their drift velocities. For electric field strengths that are relatively low, an ion's drift velocity depends on the applied electric field strength and the mobility, K, which is independent of the applied electric field and experimentally determined. The ions travel through a bath gas at a sufficiently high pressure that allows the ions to rapidly achieve a constant velocity when driven by the force of a constant electric field, which contrasts to migration in a mass spectrometry where ions accelerate in a constant electric field at low pressure.
At high electric field strengths the ion drift velocity is not directly proportional to the applied field and the mobility, Kh, is not a constant, but rather dependent on the applied electric field. This dependence has been exploited to develop high field asymmetric waveform ion mobility spectrometry (FAIMS) where ions are separated by a difference in their mobility at high field strength, Kh, relative to their mobility at low field strength, K. In FAIMS, ions are separated due to the dependent behavior of Kh as a function of the applied electric field strength.
A FAIMS spectrometer has an analyzer region defined by the space between two electrodes. One electrode is maintained at a selected dc voltage, often at ground potential, while the second electrode has an asymmetric waveform V(t) imposed upon it that is a repeating pattern of a short, th, high voltage, Vh, periods and longer, t1, lower voltage, V1, periods such that Vhth+V1t1=0 for each complete cycle of the waveform. The peak voltage during the high voltage portion of the waveform is called the “dispersion voltage” or DV.
Ions to be separated are entrained in a stream of gas flowing through the FAIMS analyzer region. The net motion of an ion is the sum of an axial x-axis component due to the stream of gas and a transverse y-axis component due to the applied electric field. The distance traveled by an ion during the high voltage portion of the waveform is given by dh=KhEhth where Eh is the applied field. Likewise, during the low voltage portion of the asymmetric waveform, d1=KE1t1. As Ehth and E1t1 are equal in magnitude the net displacement along the y-axis occurs because of the difference in Kh and K. This transverse drift is compensated by applying a constant voltage to the first electrode, the “compensation voltage” or CV. Hence, where multiple ions are present, only an ion whose drift is compensated can arrive at a detector for an appropriate combination of DV and CV. Analysis can be carried out by changing CV over time.
Various designs using curved electrode bodies have been disclosed. Buryakov et al. Int. J. Mass Spectrom. Ion Processes, 128, 143 (1993) disclosed the first FAIMS device with planar electrodes. The electric field between the planar electrodes is uniform, allowing ions to diffuse laterally. Because there is a lack of ion focusing, poor ion transmission into the narrow outlet, which is often the entrance to a mass spectrometer, affects sensitivity. The use of curved electrodes produces a two-dimensional atmospheric pressure ion focusing effect that achieves greater ion transmission efficiencies. For example, Carnahan et al. U.S. Pat. No. 5,420,424, describes a device where two cylindrical electrodes are used where one electrode is concentrically located within a tubular electrode and the ions are transmitted parallel to the central axis of the cylinders. Guevremont et al., WO 00/08455, describe a domed-FAIMS analyzer where a cylindrical inner electrode has a curved surface terminus proximate an ion outlet orifice to an analyzer region. The application of an asymmetric waveform to the inner electrode has an additional ion-focusing action that extends around the spherically shaped terminus of the inner electrode that causes the selected ions to be directed radially inwardly within the region proximate the inner electrode terminus. The inward drift is balanced by the force of the carrier gas flow and the focusing action of the applied electric fields to effectively capture the selected ions. When all forces acting upon the ions are balanced, the ions are effectively accumulated near the terminus of the inner electrode by forces of the flowing gas, or by the focusing effect of the electric fields of the FAIMS mechanism. This three-dimensional ion trap can be used in a near-trapping mode as disclosed in Guevremont et al., WO 00/08457, where the accumulated ions are leaked to an outlet orifice by a flow of gas towards the ion-outlet orifice as a narrow collimated beam where the gas flow is induced by a smaller orifice leading to the vacuum system of a mass spectrometer. This tandem domed-FAIMS/MS device is capable of detecting and identifying ions at part-per-billion levels.
Guevremont et al., WO 1/69216 disclose a “side-to-side” FAIMS. In this design the ions are transmitted around the circumference of the inner cylindrical electrode. The ion inlet and the ion outlet of a side-to-side FAIMS device are disposed, one opposing the other, within a surface of the outer electrode. An ion is selectively transmitted through the curved analyzer region between the ion inlet and the ion outlet along a continuously curving ion flow path perpendicular to the central axis of the cylinders. The ions travel approximately fifty percent of the circumference of the inner electrode and are partitioned between two streams traveling in opposite directions around the inner electrode, effective reducing the ion density within the analyzer region, reducing the ion-ion repulsion space charge effect, and allowing a reduction of the travel distance to improve the ion transmission efficiency. However, in this design the ions are not focused in a direction parallel to the central axis of the cylindrical electrodes and an inner cylinder with a small radius is required to produce a strongly focused field, which can result in ion transit times that are insufficient to separate mixtures of different ions.
Guevremont et al. U.S. Pat. No. 6,713,758 discloses a spherical side-to-side FAIMS that overcomes many of the limitations of the cylindrical side-to-side FAIMS. In this design, the cylindrical electrodes are replaced with an outer electrode with a spherical cavity and an inner electrode that is a sphere. Unlike the cylindrical side-to-side design, where the electrical field varies radially as a function of 1/r, the spherical design has improved focusing capabilities as the electric field varies as a function of 1/r2. The spherical design restores the advantage of the domed-FAIMS analyzer absent in the cylindrical side-to-side FAIMS, because the gases converge to the ion outlet and all of the ions travel a nearly identical distance from the inlet to the outlet.
Nevertheless, the spherical side-to-side FAIMS has limitations that have hindered its commercial development as design and construction of a practical spherical cell is a daunting task. For example, the suspension of a central spherical electrode exactly at the center of a spherical cavity while delivering several thousand volts of RF to the central electrode is extremely difficult. Many designs fail to provide the precise, accurate, and rigid centering required of the electrode.